Such a vessel is known from US patent application no. 2003/0005875. This document discloses a dynamically positioned vessel moored via a cavity in its hull to a releasable submerged mooring buoy. A hoisting device, comprising an A-frame lowers from the side of the vessel a light-weight pulling line that can be attached to the submerged buoy. At the end of the pulling line a connection unit is attached that can be coupled to the buoy, as well as a hawser extending through the moompool of a turret to a winching device at the top of the turret, on deck of the vessel. When the buoy is coupled to the connection device, the pulling line is released and the buoy is raised by use of the hawser and the winching device to connect to a cavity at the lower end of the turret. The A frame of the hoisting device can be maneuvered via a hydraulic cylinder for proper positioning and the winch carrying the pulling line is heave-compensated. During connection, the winching device may be subject to large forces induces by the heave moion of the vessel and the buoy. The lifting capacity of the known winch at the top of the turret needs to be large, for instance 1000 tons or more, in order to lift the buoy to its locking position in the cavity, and will require significant deck space.
From U.S. Pat. No. 7,614,927 floating production storage and offloading (FPSO) vessel is known, having in its hull a cavity in which a riser supporting buoy is connected to a cylindrical shaft in the vessel. The buoy has positive buoyancy that, in the disconnected state of the buoy, keeps it afloat at a depth below the wave active zone and is connected to a lifting cable that is attached to a winch on the vessel. The lifting cable runs along a pulley and reversing pulley of a hydraulic heave compensator comprising a horizontally placed cylinder situated on deck beside the moonpool. The heave compensator keeps the lifting rope tight, preventing it from going slack when the vessel moves up and down on the waves. The riser supporting buoy is lifted upwards via the winch until it locks into position in the moonpool via guiding and locking arms, while the vessel is kept in position by a dynamic positioning system. The known heave compensator takes up a relatively large deck space. During connecting of the buoy to the locking arms, the known heave compensating system will be deactivated to prevent relative motion of the buoy and the vessel, which may result in large forces acting on the pick up cable and on the winch.
From U.S. Pat. No. 7,513,208 a disconnectable mooring system for offshore structures such as FSO's, FPSO's, LNG regas import terminals or LNG transport vessels is known comprising disconnectable mooring buoy that is provided with a retrieval system and an intermediate buoy support equipment for detachably connecting the buoy to the bottom of a turret assembly that is rotatably positioned in the hull. A pull-in line extends from the buoy to a winch on the vessel, via a hollow bore hydraulic cylinder that comprises a rotatable hydraulic chain jack. The lifting cable near the buoy is formed by a chain section, which engages, when the buoy is pulled upward against the hull, with the hydraulic chain jack. The hydraulic cylinder assembly supports the load of the buoy while the turret assembly is rotated such that the piping on the turret is brought into proper alignment with the risers on the buoy. After alignment, the hydraulic cylinder raises the buoy at least 1 m into a position adjacent to the keel of the vessel where it is locked, after which the buoy is dewatered, the turret-buoy piping is connected and ropes and chains are stored for future buoy disconnection. The known system may during connection of the buoy be subject to large forces acting on the pull-in line caused by heave movements of the vessel.
It is an object of the present invention to provide a vessel having a lifting system, in particular a mooring system, in which a submerged structure can be rapidly connected to the vessel while forces exerted on the lifting cable caused by heave movements are reduced. It is in particular an object to provide a lifting system allowing a transition from motion compensated lifting to a lifting mode exerting a large force in the last part of the lifting trajectory, for instance when the object is raised near to or above water level, or for providing a preload of the structure against the vessel. It is a further object to provide a vessel having a heave compensator of improved design that takes up relatively little deck space and that allows rapid and controlled connection. It is another object of the invention to provide a heave compensator which reduces cable movements during heave compensation and which has an improved efficiency.